Because of differences in the operation of the inverting

regulator, the standard design procedure is not used to

select the inductor value. In the majority of designs, a 33 µH,

3A inductor is the best choice. Capacitor selection can also

be narrowed down to just a few values.

This type of inverting regulator can require relatively large

amounts of input current when starting up, even with light

loads. Input currents as high as the LM2591HV current limit

(approx 4A) are needed for at least 2 ms or more, until the

output reaches its nominal output voltage. The actual time

depends on the output voltage and the size of the output

capacitor. Input power sources that are current limited or

sources that can not deliver these currents without getting

loaded down, may not work correctly. Because of the rela-

tively high startup currents required by the inverting topology,

the delayed startup feature (C1, R

11 is recommended. By delaying the regulator startup, the

input capacitor is allowed to charge up to a higher voltage

before the switcher begins operating. A portion of the high

input current needed for startup is now supplied by the input

capacitor (C

capacitor can be made much larger than normal.

lNVERTING REGULATOR SHUTDOWN METHODS

To use the ON /OFF pin in a standard buck configuration is

turn regulator ON, pull it above 1.3V to shut the regulator

OFF. With the inverting configuration, some level shifting is

required, because the ground pin of the regulator is no

longer at ground, but is now setting at the negative output

voltage level. Two different shutdown methods for inverting

regulators are shown in Figure 12 and Figure 13

10129339

This circuit has hysteresis

Regulator starts switching at V

Regulator stops switching at V

FIGURE 10. Undervoltage Lockout with Hysteresis for Inverting Regulator

10129340

C

IN

— 68 µF/25V Tant. Sprague 595D

470 µF/50V Elec. Panasonic HFQ

C

OUT

— 47 µF/20V Tant. Sprague 595D

220 µF/25V Elec. Panasonic HFQ

FIGURE 11. Inverting −5V Regulator with Delayed Startup

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